Fluidized bed combustion apparatus

ABSTRACT

A fluidized bed furnace and boiler for the combustion of flyash with a moisture content of 45% by weight. The flyash is produced as a by-product of a coal gasification process and transported in water to a slime dam. From the slime dam a slurry of flyash and water is initially thickened in a gravity thickener (1) (FIG. 1) and then converted to filter cake in a tubular filter press (4). The filter cake is fed through pressurized hoppers (8) and chutes to the base of the boiler (10) and is distributed across the floor through chutes in the side walls and through chutes surrounded by a tubular array of water tubes and extending into the boiler furnace chamber terminating approximately one quarter of the furnace width from the side walls. A fluidized bed with a depth of about 3 meters is formed by discharging air heated in an associated air heater through nozzles in the floor of the furnace chamber to effect combustion. The combustion gases are utilized to produce steam in the boiler and are passed through a grits separator (32) and bag filter ( 34) before discharge through a flue (36).

DESCRIPTION

This invention relates to fluidised bed combustion apparatus and, moreparticularly, to apparatus adapted to consume a slurry of water andflyash with a carbon content of around 45% by weight.

Such a flyash is produced as a by-product of a coal gasification processand commonly is formed into a slurry for ease of removal and disposal inslime dams. Upon recovery from the slime dams, the flyash is in the formof a slurry with a high water content and as such poses difficulttransportion and disposal problems.

In a fluidised bed combustion apparatus arranged to be fired by flyashaccording to the present invention there is provided a furnace chamberhaving tubulous walls, floor and roof connected to extend betweendistributor means subjacent the base of the furnace chamber andcollector means adjacent the roof of the furnace chamber, a lateral gaspass extending from an upper region of the furnace chamber containing avapour generating tube bank extending between a lower, liquid, drum andan upper, vapour and liquid, drum connected to the collectors, a downpass extending from the lateral gas pass containing an economiser tubebank and an airheater and having at the base thereof a grits hopper, thedownpass discharging through a bag filter means to a flue, flyash firingmeans including chutes discharging through ports in the furnace chamberwalls closely superjacent the floor and inclined chutes extendingthrough the furnace chamber walls at an intermediate level to dischargeclosely superjacent the floor at locations spaced from the walls, theinclined chutes being surrounded by tube lengths extending from thefloor connected into respective tubulous walls, and a windbox subjacentthe floor discharging fluidising air from the airheater through nozzlesextending through the floor.

The invention will now be described, by way of example, with referenceto the accompanying, partly diagrammatic, drawings, in which:

FIG. 1 is a side elevation of the plant, including a boiler;

FIG. 2 is a sectional side elevation of the boiler;

FIG. 3 is a portion of the boiler to an enlarged scale; and

FIG. 4 is a cross-section taken on the line IV--IV of FIG. 3.

As shown in the drawings, a flyash slurry, produced in the gasificationof coal using the Koppers-Totzek process in ammonia manufacture, isfirst fed to a conventional gravity thickener 2 in order to increase itsdensity such that it then may be fed economically to a pressure filter 4of a tubular type operating at 100 bar to produce a filter cake of lowmoisture content, about 40% by weight, having good handling and storageproperties.

Upon discharge from the filter the cake is transported by means of draglink and pneumatic conveyors 6 to be pressurised feed hoppers 8 locatedadjacent an upper region of a fluidised bed fired boiler 10. Filter cakeis admitted to the hoppers through rotary valve seals 12 and meteredinto a furnace chamber 14 of the fluidised bed fired boiler by means ofdrag link feeders 16.

The furnace chamber 14 of the boiler 10 is of rectangular cross-sectionin plan, having side walls 18 of greater width than the front and rearwalls 20, 22. A combustion gas outlet 24 is formed in the upper part ofthe rear wall and discharges to a lateral gas pass 26 connected, througha 90° bend 28, to a down pass 30 having at the base thereof a gritshopper 32 which discharges, through a bag filter 34, to a flue 36. Thefurnace chamber is lined with contiguously welded finned tube panels 38extending upwardly from distributors 40 at the respective bases of thefront wall 20 and the side walls 18. A first group of tubes 42 extendsupwardly in the front wall from a first distributor 40 at the basethereof to an intermediate level where the tubes are bent inwardly andupwardly to form an intermediate front roof portion 44 and then bentagain to form an upper wall portion 46. At the top of the furnacechamber the tubes of the upper wall portion are bent to form an upwardlysloping roof portion 48 discharging to a steam and water drum 50,situated above the lateral gas pass 26 and provided with steam and waterseparating means (not shown). A second group of tubes 52 extends from asecond distrib 40 at the base of the front wall 20 to the base of therear wall 22 as a sloping floor 54 to the furnace chamber. At the baseof the rear wall, the tubes are bent upwardly to form the rear wall. Atthe level of the intermediate front roof portion 44, alternate tubes arebent inwardly and upwardly out of the plane of the rear wall to form anose arch 56. The tubes are further bent upwardly and outwardly acrossthe lateral gas pass 26 and discharge to the steam and water drum 50.Groups of tubes 58 extend upwardly in the side walls 18 from thedistributors 40 at the bases thereof to collectors 60 adjacent theintermediate roof portion 44 and adjacent the furnace chamber roof 48,which collectors are connected to discharge to the steam and water drum50. A water drum 62 is positioned at the base of the lateral gas pass26, subjacent the steam and water drum 50, and an evaporator tube bank64 extends upwardly across the lateral gas pass between the two drums.

An economiser tube bank 16 is positioned in the down pass 30 superjacentan air heater 68 provided with by-pass dampers 70.

Pendant superheater platens 72 positioned superjacent the nose arch 56are connected to receive steam from the steam and water drum 50 anddischarge steam through a superheater steam main 74.

The walls of the furnace chamber below the level of the intermediatefront roof portion 44 and the nose arch 56 are lined with refractorymaterial and are formed with penetrations for lighting-up burners 76 andfor filter cake discharge. The filter cake hoppers 8 discharge, throughvariable speed drag link feeders 16, to chutes 78 which each bifurcateat an upper level to form pairs of discharge chutes 80, 82 respectivelydischarging through inclined portions terminating in ports 84 in theside walls at the level of the furnace chamber floor 54 and extendingdownwardly at an acute angle through the side walls 18 at the level ofthe intermediate front roof portion 44 to a level adjacent the furnacechamber floor discharging through outlets 86 spaced inwardly of the sidewalls at approximately one quarter of the furnace width. The inclineddischarge chutes 82 extending into the furnace chamber are provided withtubulous water cooling means consisting of eight tubes 88 extendingalongside each chute connected in pairs into bifurcations 90, 92 at theupper and lower ends thereof. The lower bifurcations 92 are connected tofour riser tubes 94 extending through the furnace chamber floor 54 andthe upper bifurcations 90 are connected to four tubes 96 which are thenbent to extend in the plane of the adjoining side wall 18. Strips 98 arewelded intermediate adjacent tubes 88 such that a tubular enclosure isformed around the associated chute 82, which is supported and located onupper and lower groups of four wear pads 100 welded to four of the eighttubes 88. Air discharge nozzles 101 are positioned in the chutesadjacent transitions from upright to inclined portions to assist thedischarge of filter cake into the furnace chamber.

The base portion of the furnace chamber 14, below the sloping floor, isformed as a windbox 102 partioned by two laterally extending divisionwalls 104 into three, approximately equal, sections connected by ducts106 provided with control dampers (not shown) to a forced draught fan(not shown) through the air heater 68. Air nozzles 108 are positioned atspaced intervals penetrating the contiguously welded fins between thetubes forming the furnace floor for the discharge of air from thewindbox into the base of the furnace chamber to form a fluidised bed.

The boiler is top-supported by means of slings 110 depending from asteelwork frame 112.

In operation, flyash slurry from the gravity thickener 2 is partiallydried, to a moisture content of approximately 40% by weight, in thetubular press filters 4 and fed, through crushers, to the hoppers 8adjacent the boiler by means of the drag link and pneumatic conveyors 6,the rate of feed being controlled in accord with signals from high andlow level sensors on the hoppers. The partially dried flyash filter cakeis discharged from the base of each hopper 8 at a controlled rate usingthe associated variable speed drag link feeder 16, into the respectivechute 78. The filter cake is discharged into the furnace chamber,adjacent the sloping floor 54, with the assistance of air dischargethrough the nozzles at the upper ends, and at locations spacedapproximately at one quarter of the chamber width. Combustion occurs ata temperature of approximately 1000° C. and 25% excess air in the bed inthe furace chamber fluidised to a bed depth of approximately threemeters. Combustion is substantially completed in the gases dischargedfrom the bed in the remaining portion of the furnace chamber and thecombustion gases are discharged over the pendant superheater bank 72,through the lateral gas pass 26, over the evaporator tube bank 66,through the down pass 30 and grits hopper 32 to the bag filters 34 andflue 36.

Start-up is effected by fluidising the portion of the bed adjacent thefront wall 20 and firing the four lighting-up burners 76 to raise thebed temperature to a temperature at which combustion of the flyashfilter cake is self sustaining. Slurry is then fed through the twochutes 78 adjacent the front wall and the bed conditions of the portionstabilised. The central portion of the windbox 102 is then supplied withair to fluidise the superjacent portion of the bed and filter cake feedcommenced through the two chutes 78 associated therewith. Finally, uponthe front and central portions of the bed reaching stable conditions,the portion of the windbox 102 adjacent the rear wall 22 is suppliedwith air to fluidise the rear portion of the bed and filter cake feedcommenced through the two chutes 78 adjacent the rear wall 22 until thecomplete bed is operating stably.

Part load operation of the bed is achieved by slumping one or two of theportions of the bed by closing the control dampers in the ductssupplying air to the respective portions of the windbox 102.

Heat transferred from the combustion gases to the various tube surfacesserves to generate and superheat steam for utilisation elsewhere in theplant.

We claim:
 1. A fluidised bed combustion apparatus arranged to be firedby flyash including a furnace chamber (14) having tubulous walls (18,20, 22), floor (54) and roof (44, 48) connected to extend betweendistributor means (40) subjacent the base of the furnace chamber andcollector means (60) adjacent the roof of the furnace chamber, a lateralgas pass (26) extending from an upper region of the furnace chambercontaining a vapour generating tube bank (64) extending between a lower,liquid, drum (62) and an upper, vapour and liquid, drum (50) connectedto the collectors, a down pass (30) extending from the lateral gas passcontaining an economiser tube tank (16) and an airheater (68) and havingat the base thereof a grits hopper (32), the downpass dischargingthrough a bag filter means (34) to a flue (36), characterised in thatmeans for firing the flyash include chutes (80) discharging throughports in the furnace chamber walls (18) closely superjacent the floor(54) and inclined chutes (82) extending through the furnace chamberwalls (18) at an intermediate level to discharge closely superjacent thefloor (54) at locations spaced from the walls, the inclined chutes beingsurrounded by tube lengths (88) extending from the floor (54) connectedinto respective tubulous walls (18), and a windbox (102) subjacent thefloor discharging fluidising air from the airheater (68) through nozzles(108) extending through the floor to form a fluidised bed of the flyash.2. A fluidised bed combustion apparatus as claimed in claim 1,characterised in that the flyash in the form of a slurry consisting of amixture of water and flyash is arranged to be partially dried in tubularpresses (4) prior to discharge to the furnace chamber (14).
 3. Afluidised bed combustion apparatus as claimed in claim 2, characterisedin that the chutes (80) and inclined chutes (82) are connected toreceive the partially dried flyash from pressurized hoppers (8) throughenclosed, cariable speed, drag link feeder means (6).
 4. A fluidised bedcombustion apparatus as claimed in claim 3, characterised in that thepressurized hoppers (8) are connected to receive the partially driedflyash through rotary valves (12).
 5. A fluidised bed combustionapparatus as claimed in claim 3, characterised in that each pressurizedhopper (8) has associated therewith one chute (80) and one inclinedchute (82) discharging at locations equidistant from the furnace chamberfront wall (20).
 6. A fluidised bed combustion apparatus as claimed inclaim 1, characterised in that the windbox (102) is formed as aplurality of compartments by division walls (104), each compartmentbeing connected to receive air from the air heater through respectivedamper controlled ducts (106).
 7. A fluidised bed combustion apparatusas claimed in claim 6, characterised in that each compartment hasassociated therewith two pressurized hoppers (8), one to each side ofthe furnace chamber (14), each pressurized hopper having associatedtherewith one chute (80) and one inclined chute discharging at locationssuperjacent the compartment.
 8. A fluidised bed combustion apparatus asclaimed in claim 6, characterised in that lighting-up burners (78) arepositioned in the furnace chamber walls (20) to discharge into a furnacechamber region superjacent one of the windbox compartments.
 9. Afluidised bed combustion apparatus as claimed in claim 1, characterisedin that the tubulous walls (18,20,22), floor (54) and roof (44,48) ofthe furnace chamber are formed from contiguously welded finned tubepanels (38).
 10. A fluidised bed combustion apparatus as claimed inclaim 9, characterised in that the nozzles (108) extending through thefloor extend through and are welded to finned portions of the respectivepanels.